Color differs from Cycles render in Video Sequence Editor
You definitely have an issue in color management, specifically regarding transfer curves, but from the information you provided it is impossible to conclusively explain exactly why or how.

A PNG, if encoded correctly, should display as it was encoded in the VSE. If you somehow managed to mangle up the transfer curve when storing it in the PNG, via the "View as Render" awfully named checkbox for example, all bets are off.

So to properly diagnose this:
1. What were your render settings with regards to color management? Did you happen to check the "View as Render" toggle somewhere?
2. What file format did you save and how? For example, is it possible the file format was saved differently and merely labelled with the PNG extension?
3. What are the color space settings for both render and the VSE?